When we choose between two options based on the rewards they promise, we make use of a highly specialized set of brain circuitry. Understanding this circuitry is important because it is the major target of abused drugs and because dysregulation of reward-based decisions is a major symptom of drug addiction. The goal of this project is to understand how that circuitry works. We focus on one brain area, the ventromedial prefrontal cortex, that is consistently identified by neuroimaging studies as a critical site for rewards and decisions, but whose function remains mysterious. To probe its function, we will record responses of single neurons in this area while subjects perform two different tasks specifically designed to probe choice processes. First, we will examine the representation of risky options, and see whether vmPFC neurons represent their components - risk, stakes, probability, separately, or in a unified framework. Second, we will see whether neurons in this area signal positive and negative offers on a common scale. These first two projects will tell us how information about rewards is represented by vmPFC neurons. Then we will examine the responses of these neurons in a simple decision-making task in which subjects can accept or reject offers that bring costs and benefits. Finally, we will test whether vmPFC plays a causative role in choice by artificially altering neuronal activity there with a stimulating electrode. After completing these studies, we will have a first-ever basic understanding of the neuronal mechanics of reward-based choice processes.